Electric devices, such as headlamps, tail lamps, fog lamps, power windows, pressure sensors, pressure switches, and engine control units of automobiles, are used in an environment in which they are exposed to liquid, such as water, oil, and surfactants. Furthermore, also among general home appliances, electric razors, mobile phones, electric toothbrushes, and others are used in an environment in which they are exposed to liquid, such as water, oil, and surfactants. In these electric devices, a case for putting electronic components therein needs to be highly droplet-proof. If, however, the case is completely airtight, the expansion and contraction of gas in the case due to temperature changes causes a difference in pressure between the inside and outside of the case, and excessively encumbers the case. Accordingly, the case needs to remain capable of allowing gas to enter and exit from it, while preventing liquid from entering.
Patent Document 1 describes a vent cap including: a cover component having a cylindrical shape with a bottom; and a cylindrical body fitting within the cover component. A labyrinthine vent passage is formed between the inner periphery of the cover component and the outer periphery of the cylindrical body, and also between the bottom surface of the cover component and the bottom portion of the cylindrical body, whereby the vent cap exhibits waterproofness and air permeability.
Patent Document 2 describes a ventable plug that is attached to a case of an electric motor for an automotive power window. In a sheet joining portion of this cylindrical plug body, an air-permeable sheet is formed so as to close the inside of the plug body, whereby water is prevented from entering the motor case, and the function of the air-permeable membrane allows ventilation between the inside and outside of the case.
Patent Document 3 describes a motor having a breathing hole, to a cylindrical portion of which an attachment member is fit so as to be attachable thereto and detachable therefrom, the attachment member obtained by forming, in an attachment body, partially protruding blockage prevention protrusions on an edge surface around an opening at one end of the attachment body formed of an elastic member such as a rubber material; and fixing a waterproof sheet so as to cover the opening of the attachment body. The attachment member can prevent water such as rainwater from entering the breathing hole. Further, the presence of the protrusions prevents an opening portion of the breathing hole from being blocked by contact with the surface of another member.